This invention relates to methods and arrangements for development of two or more electrostatic images in succession on a substrate.
In some electrostatic image development systems such as multicolor imaging systems, the surface of an electrophotographic member is provided with a uniform charge and subsequently exposed to form a first electrostatic image which is then developed with one developer, and the same surface is again charged and exposed for a different image which is then developed with a different developer, and the same process may be repeated for still another image developed by a still different developer before the combined images are transferred to a permanent support member.
In image forming systems of this type, there is a problem resulting from undesired removal of toner from the surface of the photoreceptor which has been deposited by development of a previous image when a subsequent image is being developed. This not only degrades the first image, but also contaminates the developer used to develop the later image.
Several different approaches have been attempted to overcome this problem. For example, U.S. Pat. No. 5,176,487 discloses an arrangement in which the first image is developed by a one-component magnetic developer and the second image is developed by a two-component developer with a non-magnetic toner and a magnetic carrier which is the same as the one component developer used to develop the first image. Consequently, any of the one-component developer removed from the first image during the second development merely acts as a carrier in the second developer unit and does not contaminate the second developer.
In U.S. Pat. No. 5,418,097 noncontact development of successive color images is effected by imposing an AC voltage with a DC bias between a member bearing an electrostatic image and a developer roll spaced from the image-bearing member and carrying the toner to be applied to the image, causing toner to jump to the electrostatic image on the image-bearing member. According to that patent, the time during which toner is attracted from the developer roll to the image during each AC cycle should be greater than the time during which toner might be attracted from the electrostatic image to the developer roll.
U.S. Pat. No. 5,532,801 discloses a multi-image developing arrangement using an AC toner transfer voltage with a DC bias applied between the developer roll and the image-bearing member in which the rate of change of the AC voltage is reduced during the time when the AC voltage changes from the toner transfer voltage to a voltage at which toner will be returned to the developer roll and is increased during the time when the toner transfer voltage is changing from the toner return voltage to the toner transfer voltage. This reduces the tendency of toner on the image-bearing member to return to the developer roll.
U.S. Pat. No. 5,890,038 describes a two-color image forming apparatus in which the toner particles in the developer unit which forms a first toner image on an image-bearing member spaced from the developer roll are larger than those of the developer used to form a second image on the image-bearing member. As a result, the electrostatic force with which the first toner particles are retained on the developer roll of the second developer unit is lower than the force with which the particles of the second toner are retained on the developer roll of that developing unit, permitting the first toner particles in that unit to be removed from the developer roll.
Such prior art attempts to avoid the problem of removal and mixing of toner from one developed image during development of a second image on the same substrate involve complex procedures or structural arrangements and in some cases require different types of developers for the different images.